Apparatus and method for starter RPM detection

ABSTRACT

An engine starter motor tester applies power to a motor under test at the level normally applied while that motor is installed in a vehicle and monitors the ripple in the driving current. The ripple rate, scaled by the number of windings used in the construction of the motor, provides a direct reading of the shaft speed of the unloaded motor. This number allows a valid inference concerning the state of health of the motor, and is more certain than an estimate for an unloaded motor based on operator experience and motor “sound”. A complete test apparatus includes a stored lookup table, allowing the user to key in a product brand and model number and read directly both the RPM and the test apparatus&#39;s rating of the motor under test, expressed in terms such as percentage of life remaining.

FIELD OF THE INVENTION

The present invention relates generally to vehicular and non-vehicularengine starter motor test equipment. More particularly, the presentinvention relates to an apparatus and method for testing a starter motorby measurement of rotational speed under power.

BACKGROUND OF THE INVENTION

Engine-powered vehicles, recreational equipment, and machines commonlyuse battery-powered electric motors to start their engines. Such startermotors have a variety of failure modes. A typical maintenance procedurewhen starter motor failure is suspected involves removing the motor andtesting it using dedicated test equipment. Such a unit of test equipmenttypically consists of a bracket to which the motor under test can beclamped, a wall-plug-AC-powered DC power supply, heavy-duty cables tofeed the power to the motor under test, and a variety of connection andcontrol schemes to allow any of the various styles of starter motors tobe actuated in a way similar to their normal on-engine actuation modes.

In pursuit of minimization of complexity and component cost, typicalstarter motor testers employ simple power supplies and provide no directindication of the condition of the starter motor, relying on theexperience of the operating technician to decide whether the sound ofthe running starter motor is “about right”. While effective fordiagnosing simple failures such as worn-out brushes, shortedcommutators, open field windings, and failed hold coils, and to verifythat a replacement starter motor acts nominally as part of a salestransaction, such tester designs may fall short when testing newerstarters. For example, recent starter motor designs can differ greatly,so that technician experience may be less reliable as a guide to motorcondition. Meanwhile, starter motors for fossil/renewable hybrid and lowemissions vehicles may be used at virtually every vehicle stop, sodemands for starter motor reliability and durability can increase, whichcan in turn dictate a need for higher analytical precision in a tester.Conventional tester designs may also fail to take advantage of recentinnovations in electronic components and concepts that promisesignificant benefits, including the capability to detect failure modesthat may be partially masked when starter motors are run without a load.

Incorporating into a starter motor tester the capability to apply amechanical load simulating an engine sized for each motor has presenteddisadvantages. The variety of dimensions of run-away clutches, as wellas the variety of mounting bracket styles used to affix starter motorsto engines, makes the mechanical connection of a starter to a loadsimulator a significant challenge. Such a load simulator, in turn, wouldhave significant technical challenges to its implementation in supportof all likely engine types, from motorcycles to heavy trucks.Temperature testing to verify a starter motor's ability to function in aworst case would be desirable, but would require chilling the startermotor to simulate extreme winter conditions, which presents thedisadvantage of complexity.

Accordingly, there is a need in the art for a starter motor tester thatprovides increased test capability compared to conventional designs.

SUMMARY OF THE INVENTION

A preferred embodiment of a starter motor tester comprises a mechanicalclamping apparatus to restrain a motor under test from rotating duringstartup and slowdown; an electrical circuit to apply power to thestarter motor; a detector circuit to sense the rate at which the startermotor spins; a current monitor circuit to detect the instantaneouscurrent flowing through the motor under test; a current scaler toconvert the detected level to a form usable by the test circuitry; arate sense function to detect the ripple frequency of the load current;a rate scaler to adjust the signal from the rate sense circuit accordingto the properties of the motor under test; a display driver circuit toconvert the scaled rate signal to a human-readable form; and a displayto report the detected spin rate (commonly termed RPM in reference tothe standard unit of measure, revolutions per minute).

In accordance with another embodiment of the present invention, anapparatus for performing starter motor testing tasks establisheselectrical connection between a suitable source of DC power and astarter motor being tested and applies that DC power to the motor. Itfurther applies control signals to the starter motor, senses the currentlevel drawn by the starter motor, and detects pulsation in the sensedstarter motor current level attributable to addition and removal ofindividual motor windings from the power circuit as the motor's brushesmake and break contact with the individual segments of the motor'sarmature. It further converts detected pulses into a pulse rateindication, adjusts the pulse rate indication based on properties of theindividual starter motor in order to determine the RPM of the startermotor under test, and displays as an output the sensed and scaled RPM ofthe starter motor under test.

In accordance with another embodiment of the present invention, a methodfor testing vehicle engine starter motors features the steps ofaccepting AC input power from public utility AC mains; converting theincoming AC input power to DC at the range of levels of voltage andcurrent required for operating engine starter motors; applying the DC tothe starter motor under test; sensing the average and transient currentof the power applied to the starter motor under test; converting theripples in the sensed current level to a ripple rate; scaling the ripplerate signal based on the number of windings and/or any gear reductioncharacteristic of the starter motor under test to yield an effective RPMvalue; displaying the effective RPM value in human-readable form;displaying a summary indication of the condition of the starter motorunder test; and providing electrical connectivity to integrate thestarter motor testing method and the starter motor under test.

There have thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described below andwhich will form the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments, and of being practiced and carried out in various ways. Itis also to be understood that the phraseology and terminology employedherein, as well as the abstract, are for the purpose of description, andshould not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a mechanical arrangement suitable forimplementing the preferred embodiment of a starter motor tester.

FIG. 2 is a block diagram illustrating a starter motor tester thatincorporates an RPM sensor according to a preferred embodiment of thepresent invention.

FIG. 3 is a first schematic diagram showing the sensor configurationused in the preferred embodiment of the invention, in which areturn-line current monitor and differentiator are used to implement theRPM sense circuit.

FIG. 4 is a “screen shot” representing an oscilloscope screen showingthe waveforms characteristic of a signal from a current sensor and froma Hall effect probe operating at the same rate.

FIG. 5 provides a flowchart illustrating the steps performed by apreferred embodiment of a starter motor tester.

FIG. 6 is a second schematic diagram of an embodiment similar to FIG. 2with the exception that a magnetic field sensor is employed in place ofa current sensor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A preferred embodiment of the invention provides a vehicle enginestarter motor tester that includes a safety clamp to restrain the motorfrom moving during testing. A power source applies the equivalent ofvehicle battery power to a subject motor. Interconnect cables, clamps,and switching circuitry allow multiple styles of conventional startermotors to be powered. A current monitor senses and displays the draw ofthe subject motor for comparison to the standard for that motor. Also,in the preferred embodiment of the invention, a detector and itsassociated circuitry sense and display motor RPM during the test.

FIG. 1 shows a starter motor test set 12 that encloses the electronicsin a simple housing 14 and restrains a starter 16 under test with aclamp 18. The test results are displayed on a readout display 20. Thetest set requires few or no controls on the panel; the preferredembodiment can use a simple test command pushbutton 22 to run the test.The power cables 40 and the associated power cable clamps 66 provideconnection for primary power. Additional interconnecting wires 68support whatever additional signals may be required for testing aspecific type of starter motor.

FIG. 2 illustrates a preferred embodiment of the present inventiveapparatus and method in block diagram form. Within a starter motortester 24, a power supply 26 that uses a protection function 28, anelectromagnetic interference (EMI) filter 30, and a main power switchingdevice 32, draws power from AC mains by a standard electrical plug 34and provides the power to AC-to-DC conversion circuitry 36 and an outputEMI filter 38.

The power supply output in the preferred embodiment provideslow-resistance electrical continuity through a pair of power cables 40and a series current sensing device 42 to the primary inputs 44 of thestarter unit under test 46. The signal developed across the currentsensing device 42 is subjected to analog signal level adjustment andfiltering 48 to produce an isolated signal of suitable magnitude forprocessing by an analog-to-digital (A/D) converter 50 whose outputdigital data stream is fed to a digital signal processor (DSP) 52. TheDSP 52 also receives data from additional digital data sources via aninterface buffer 54; in this embodiment the additional sources include aUniversal Serial Bus (USB®) port 56 that permits a lookup table ofstarter model numbers with selected characteristics of each to bedownloaded into the tester from a remote source and a keyboard 58 thatallows the user to enter characteristics for the starter type under testwithout depending on a remote source.

FIG. 2 also shows the output of the DSP 52 as a display 60 thatindicates results of the testing, for instance expected RPM, actual RPM,and average current draw. In this preferred embodiment, the DSP 52 alsosupports the keyboard 58, and can be used to generate displayinformation such starter motor data as model number, number of poles,gear reduction ratio, rated torque, and the like for presentation on thedisplay 60.

As further illustrated in FIG. 2, a typical starter motor may have twomain power conductors only (a first conductor in the form of a wire for“hot” and a second conductor in the form of ground return via the bodyof the starter is the minimum that can work; starters of this type aswell as starters with two power wires can be supported using thepreferred embodiment), or may have two main power conductors plus avarying number of additional wires; the configuration for a test dependson the number of additional components integrated into the starter unitunder test 46. A driver circuit 62 in the tester 24 allows theseadditional functions in the starter under test to be exercised withexcitation and loads comparable to those in the vehicle.

The AC/DC power supply could apply power to starters designed forso-called positive-ground systems as well as for negative-groundsystems.

The electronic circuitry in the preferred embodiment may require astand-alone power unit 64 isolated from the DC supply 26 used toenergize the starter under test 46.

FIG. 3 shows a greater level of detail in the analog portion of thewaveform analysis function. FIG. 4 shows waveforms characteristic of thecircuit designs in FIG. 3 and FIG. 6, and will be referenced during thediscussion of both of the latter figures. The voltage developed across asense resistor 70 may require amplification and buffering with anisolating amplifier 72. The amplifier 72 output may require furtherconditioning with a bandpass filter 74 to remove the DC level anduncorrelated noise, such as high-frequency switching noise from theAC/DC converter 36 in FIG. 2. The filter 74 can thus yield a series ofsignal pulses associated with the powering of successive armaturewindings through the brushes in the starter under test 46 in FIG. 2.

FIG. 4 shows a representative waveform present in thecurrent-sensor-based rate detection circuit as a waveform 76. Thevoltage signal represented by this waveform 76 may then be applied to ananalog-to-digital converter (A/D) 78 to be digitized at a sufficientsampling rate to avoid aliasing, with the sample rate under appropriatecontrol. The term “aliasing” is used here as normally used by thoseskilled in the art, namely to specify sampling at or above the Nyquistrate. In the case of a starter tester, the sample rate would ordinarilybe either twice the highest known pole rate for an unloaded starter or arate selected by the DSP 80 for each starter type, as preferred for agiven implementation. Alternative designs may be used, such as analogsystems feeding shaped pulse streams to stand-alone counter modules.

The waveform 76 thus sampled could contain a characteristic repetitivepattern. Analysis within the DSP 80 could in turn detect the pulse ratecausing this pattern, which is the rate at which poles of the starterarmature are successively energized by the starter motor's brushes. Fromthat rate and the data characterizing the selected starter, the testercan determine the rotation rate of the armature under test. Since thevalue of interest is generally the armature rate rather than the outputshaft rate, the armature pole rate may be used to indicate the conditionof the starter under test. Output shaft rate may also be a datum ofinterest.

The DSP 80 in the preferred embodiment should be able to store, manage,and upgrade a database of starter properties in internal or externalnonvolatile memory, such database of sufficient size to meet the normalrequirements of the intended product market.

An example of an alternative technology usable in place of a DSP is ageneral-purpose field programmable gate array (FPGA). Some FPGAs can beprogrammed with sufficient functionality to perform the requiredoperations, as well as to store a block of starter data in internal orexternal nonvolatile memory.

Another embodiment can use a unitized microcontroller, such as a memberof the PIC® product family, to implement a significant part of thedesign described above. Typical PIC family devices include buffering andA/D conversion, but would likely require some external passive lowpassfiltering to suppress aliasing, and would lack a large memory store tomanage a starter database internal to the PIC. Thus, an implementationwith a PIC could require an additional external nonvolatile memorystore, and would likely yield an effective product complexity comparableto the previously described embodiments.

FIG. 5 illustrates another embodiment of the present inventive apparatusin flowchart form. This embodiment provides for conversion of mains ACto a DC level suitable for vehicle engine starter motors 82 and forapplication of this power to the motor under test 84 (UUT). Theembodiment further detects pulsations in DC electric motor drive current86, converts the pulsations to a numerical rate value 88, and convertsthis number to a corresponding motor performance value 90. Theembodiment then displays the performance value 92, and rates 94 themotor under test compared to the standard for its type.

Additional manual or automatic capabilities, such as detection of thetype of starter motor, determination of starter motor requirements, andfurnishing of excitation requirements to motor drive circuitry, can beimplemented to produce a complete commercial product.

A system according to the preferred embodiment of the invention may haveother design elements, such as, for example, a housing, inputelectromagnetic interference (EMI) filter circuitry, output EMI filtercircuitry, protection circuitry to provide circuit interruption in eventof power fault, power factor correction, a user interface, a temperaturemonitor, and other design elements appropriate for industrialapplication of starter motor testing functions.

The preferred embodiment of the invention, in providing a built-in RPMscaling function, introduces a level of capability for precision whosebenefit offsets the disadvantage of adding to system complexity.Similarly, the use of a digital readout to display RPM and/or otherproperties measured during the test, can provide information to allow acredible estimate of the performance capability of the unit under testcompared to a new or ideal unit of the same type, rather than a simplego/no go observation.

Alternative embodiments can, for example, sense rotation rate directlyby detecting changes in the magnetic field outside the starter during atest. FIG. 6 shows an example of such a sensor circuit using a Halleffect device. Placed proximate to a motor under test, a sensor based ona Hall effect device could detect changes in the motor's magnetic fieldas the armature rotates. This signal, suitably buffered 100 and filtered102 using a circuit as illustrated in FIG. 6, could produce a waveform96 as shown in FIG. 4. The Hall effect component 98 of the circuit ofFIG. 6 could be an active sensor powered by the self power supply 64shown in FIG. 2. Such a device, by directly producing a usable outputsignal as each armature pole passes, could simplify DSP 104 programming.Digital interface 106 signals and display 108 characteristics could belargely unaffected by the choice of sensor technology. There are stillother existing sensing devices capable of producing accuratemeasurements.

Other alternative embodiments could include still other technologies fordetecting motor speed by detecting current drawn by individual armaturepoles, such as an inductive current sensor, clamp-on DC ammeter, or thelike, attached to or surrounding the DC power wiring, without deviatingfrom the intent of the series resistor approach already described.Similarly, a motion detector for ferrous metals or a similar device,placed adjacent to the gear teeth of the starter motor drive mechanism,could directly sense output shaft motion, detecting the achievedrotation rate of the starter and permitting its display.

In a complete implementation of the present invention, certain of thesteps making up the preferred embodiment could be performed using acomputing device, stored memory system, binary data input, and a humaninterface for commands and signals.

The application of the inventive apparatus in the testing of motorsmanufactured for use in starting internal combustion engines in vehiclesdoes not exclude its further application for testing other types ofmotors, such as those used for starting internal combustion engines usedfor powering generators, pumps, and other stationary and portableequipment, as well as other motor types that use DC power and provideintermittent or continuous service. The features of the inventiveapparatus are further applicable to testing of motors powered from AC.

The many features and advantages of the invention are apparent from thedetailed specification, and, thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirit and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described; accordingly,all suitable modifications and equivalents may be resorted to, that fallwithin the scope of the invention.

1. A tester for vehicle engine starter motors, comprising: a mechanicalclamping apparatus to restrain a motor under test from rotating duringstartup and slowdown; an electrical circuit to apply power to thestarter motor; a data storage capability allowing entered datacharacterizing a plurality of starter motor types to be retained withinsaid starter tester; a data selection device allowing a single startermotor type to be selected from entered data; a detector circuit to sensethe rate at which the starter motor spins and comparing with a standardspin rate of a motor of the same type; and a display to report thedetected spin rate and the comparison with the standard spin rate. 2.The tester of claim 1, wherein said detector circuit further comprises:a current monitor circuit to detect the instantaneous current flowingthrough the motor under test; and a current scaler to convert thedetected motor drive current to an electrical signal characterized bytransient signal components associated with starter motor armaturerotation.
 3. The tester of claim 1, wherein said detector circuitfurther comprises: a magnetic field detection device (MFDD) external tothe motor under test to produce a sensible response to transient startermotor magnetic field effects; and an MFDD interface circuit to convertthe sensible response from the MFDD to a form usable by the displaycircuitry.
 4. The tester of claim 1, wherein said detector circuitfurther comprises: a rate sense function to detect the ripple frequencyof the load current; a rate scaler to adjust the signal from the ratesense circuit according to the properties of the motor under test; and adisplay driver circuit to convert the scaled rate signal to a formcompatible with a display device.
 5. The tester of claim 1, wherein saidelectrical circuit to apply power further comprises: an input circuitconfigured to accept alternating current input power from public utilityalternating current mains; and a power supply to convert inputalternating current to low-voltage direct current suitable forapplication to a starter motor under test.
 6. The tester of claim 5,wherein said power source supports testing of 6-volt, 12-volt, and24-volt starter motors.
 7. The tester of claim 5, wherein said powersource supports positive-ground electrical systems.
 8. The tester ofclaim 1, further comprising: a data entry device to furnish datacharacterizing a plurality of starter motor types to said startertester; and a processing device to apply controlling values to thescaling circuit of said starter motor tester based on informationextracted from entered data.
 9. The tester of claim 8, wherein said dataentry capability further comprises: a first data entry capability thatpermits entry of a store of information identifying starter motors bymodel designation associated with such properties of each model as maybe relevant to the testing; and a second data entry capability thatpermits entry into the tester of type identification for the specificstarter motor under test.
 10. The tester of claim 8, wherein said systemsupports manual entry of test parameters.
 11. The tester of claim 8,wherein the data entry device including entry of standard values for themotor, stored in the data storage, and processed by the processingdevice for comparison of the standard values with the detected values ofthe motor.
 12. The tester of claim 1, further comprising an auxiliarypower circuit configured to provide, from a source external to saidtester, power to the electronic circuitry of the tester.
 13. The testerof claim 1, further comprising a set of devices to provide electricalconnection between the test apparatus and the starter motor under test.14. The tester of claim 13, wherein said electrical connection devicessupport a connection that simulates the in-vehicle connection from thevehicle battery to the starter motor under test.
 15. The tester of claim13, wherein said electrical connection devices support a direct groundreturn through the case of the starter motor under test.
 16. The testerof claim 13, wherein said electrical connection devices support asolenoid activation terminal separate from the battery and groundconnections.
 17. The tester of claim 13, wherein said electricalconnection devices support testing the added function of starter motorsequipped with an auxiliary ignition terminal separate from any power andsolenoid activation terminals that may be present on such startermotors.
 18. The tester of claim 13, wherein said electrical connectiondevices support testing starter motors used with separate solenoids. 19.The tester of claim 1, wherein said system supports evaluation of thecondition of the start and hold coils and any contacts of starter motorsolenoids.
 20. The tester of claim 1, wherein a current sense circuitprovides a measurement of the average motor current.
 21. The tester ofclaim 1, wherein the detector circuit further comprises: a resistorreceiving input from the motor under test; an isolating amplifierconnected to the resistor with voltage developed across the resistor,including amplification and buffering from the isolating amplifier; anda filter connected to the isolating amplifier removing the directcurrent level and uncorrelated noise from the electrical circuit,yielding a series of signal pulses associated with the powering ofsuccessive armature windings in the motor under test.
 22. The tester ofclaim 1, wherein the electrical circuit to apply power, furthercomprises: a protection device receiving input from an alternatingcurrent power source and providing a protection function for thereceived power; a filter providing electromagnetic interferencefiltering of the power from the protection device; a power switchingdevice controlling the power from the filter; an alternating current todirect current device converting the power from the power switchingdevice; and a second filter providing an electromagnetic interferencefiltering of the direct current from the alternating current to directcurrent device, providing power to the motor under test.
 23. The testerof claim 1, wherein the detector circuit further comprises: a signalprocessor receiving signal from a current sensor from a return of thecurrent from the motor under test, the signal developed across thecurrent sensor is subject to signal level adjustment and filtering fromthe signal processor to provide a predetermined magnitude; an analog todigital converter receiving the signal form the signal processor andconverting the signal from analog to digital; a digital signal processorreceiving the signal from the analog to digital converter and processingthe signal for display; and an interface buffer providing information ofthe motor from a remote source to the digital signal processor forprocessing for display.
 24. The tester of claim 1, wherein the detectorcircuit further comprises: a means for detection of pulsations in themotor under testing; a means for converting the pulsations to anumerical rate value; and a means for comparing the rates of the motorunder test and a standard for the type of motor under test for display.25. The tester of claim 1, wherein the mechanical clamping apparatus,the electrical circuit, the detector circuit and display beingintegrated and housed in a single unit.
 26. An apparatus for testingvehicle engine starter motors, comprising: a clamp restraining a motorunder test from rotating during startup and slowdown; a power supplyproviding power to the starter motor at the level of a compatiblevehicle; a detector module receiving the current output form a startermotor under test, further comprising: a first module sensing the currentfrom the motor or from the magnetic field outside the motor; a secondmodule converting the signal from the first module for processing; and athird module receiving input from the second module and converting theinput received to a spin rate, and comparing the spin rate with standardspin rates of the motor received from an external input; a data storagecapability allowing entered data characterizing a plurality of startermotor types to be retained and communicatively linked to the externalinput; a data selection device allowing a single starter motor type tobe selected from entered data; and a display to report the informationoutputted from the detector module.
 27. The apparatus of claim 26,further comprising a driver circuit providing additional functions inthe motor under test to be exercised with excitation and loads of thevehicle adapted for the motor.